Optical fibers are key components in modern telecommunications systems. An optical fiber is a thin strand of glass capable of transmitting an optical signal containing a large amount of information over long distances. In essence, it is a small diameter waveguide characterized by a core with a first index of refraction surrounded by a cladding having a second (lower) index of refraction. Light rays which impinge upon the fiber core at an angle less than a critical acceptance angle undergo total internal reflection within the core. These rays are guided along the axis of the fiber with minimum attenuation. Typical optical fibers are made of high purity silica with minor concentrations of dopants to control the index of refraction.
Optical fiber filters are highly useful in multiwavelength fiber communications systems. In such systems an optical fiber simultaneously carries many different communications channels in light of respectively different wavelengths. In the use of such systems it is frequently required to add channels onto the fiber and to selectively remove channels from the fiber. Optical filters are frequently used to assist such addition and removal.
While there are a variety of methods for making optical fiber filters, most such methods require especially doped fibers and expensive equipment such as ultraviolet lasers. The most popular technique involves fabricating fibers doped to exhibit ultraviolet light photosensitivity and exposing the fiber at periodic regions along its length to high intensity ultraviolet light. This exposure produces optical index perturbations which, in short periods, can operate as reflective Bragg gratings and, in long periods, can operate as mode conversion filters. There remains a need for simpler, less expensive method for making optical fiber filters.